A wide variety of medical devices are made from thermoplastic polymers. Medical devices must be manufactured with greater care than general consumer products especially when inserted into the body or brought into contact with a wound or lesion. In the area of treatment devices, such as catheters, manufacturers must take great care to assure that the devices perform with an extremely high degree of reliability. At the same time there is a need to develop materials and improve processing techniques to obtain improvements in desirable properties such as tensile strength, flexibility, puncture resistance, and softness. One area in which development has been especially intense focuses on balloons deployed on catheters which are utilized for dilatation, especially angioplasty, for stent placement, for urinary treatment, and the like.
In preparing high strength balloons for medical devices such as dilatation and stent placement catheters, a variety of polymer materials have been used.
Levy, U.S. Pat. No. 4,490,421, describes use of PET of high molecular weight (1.0 IV or higher). The patent notes that the IV may decrease during processing into balloons. Such a decrease is believed to be related to polymer degradation caused by extrusion temperature and the time the resin is held in the melt. Lower molecular weight PET has also proven useful for preparing high strength balloons. See for instance Noddin et al, U.S. Pat. No. 4,963,313; Saab, U.S. Pat. No. 5,264,260; Wang et al U.S. Pat. No. 5,348,538.
Polyamide balloons are described in Pinchuk, U.S. Pat. No. 5,108,412. Polyurethane block copolymer balloons are described in Gahara, U.S. Pat. No. 4,950,239, and Anderson et al, U.S. Pat. No. 5,500,180. Polyamide block copolymer balloons and polyester block copolymer balloons are described in Wang et al, U.S. Pat. No. 5,563,383. Various other polymers have also been used for catheter balloons.
A wide variety of polymer blends have also been described for such balloons, for instance, Sahatjian et al, U.S. Pat. No. 5,500,180; Chen et al, U.S. Pat. No. 5,554,120, Hamilton et al, U.S. Pat. No. 5,797,877. Some such blends have included compatibility enhancing additives. However, heretofore it has not been proposed to include additives which maintain or increase molecular weight during melt processing.
A typical process for forming catheter balloons involves extruding a tube of thermoplastic polymer material from a melt composition and then blowing the extruded tube at an elevated temperature above the Tg (using highest Tg in case of block copolymers), optionally with ambient or elevated temperature stretching, to form the balloon with a radial and/or longitudinal molecular orientation. See e.g. Wang et al U.S. Pat. No. 5,348,538, Wang et al, U.S. Pat. No. 5,563,383; and Wang et al, U.S. Pat. No. 5,714,110. The ability of the extruded tubing material to successfully be processed in this manner is thus an important requirement of polymer compositions used to form catheter balloons.
For instance, in practice, the polyester PBT (polybutylene terephthalate), and some butylene terephthalate copolymers, were observed to produce extruded tubing which had a tendency to opacify and/or to resist thermal forming into a balloon configuration. In WO99/44649 boric acid is added to PBT and butylene terephthalate copolymers to improve post-extrusion processing characteristics of such polymers when forming catheter balloons.
In some thermoplastic polymer processing arts it has been proposed to maintain or increase the molecular weight of polyesters or polyamides by adding to a polymer melt a difunctional additive which can react with polymer chain ends to extend chains. Chain extending melt additives have been described for polyesters and polyamides which have a relative low tendency for crosslinking. Examples of such additives are described in U.S. Pat. No. 6,228,980, WO 96/34909, and EP 0288253. However, before the present invention, the suitability of such additives in the manufacture of medical devices has not been explored. Nor does it appear that the effects of such additives on post-extrusion sub-melt thermal formability of polymer compositions have been considered.